1. Field of the Invention
The present invention relates to an airbag cover adapted to cover a folded airbag, having a lid portion which is opened to form a projection opening through which the airbag projects when a thin breakable portion which is provided to surround the lid portion is broken by a pressure of the inflating airbag, and formed from a synthetic resin material by molding.
2. Related Art
In a conventional airbag cover which is made from a synthetic resin, a thin breakable portion was formed around a lid portion so that the lid portion could be opened to form a projection opening when the cover was pushed by an airbag (refer to, for example, JP-A-2005-289254). This breakable portion was made by providing a recessed groove on a rear surface side of the airbag cover which was injection molded, and furthermore, in order to enable the operation of forming the breakable portion with enhanced dimension accuracy as to the depth of the recessed groove so provided, the breakable portion was formed through cutting operation in which a notch was made by making use of a cutting blade of a working tool such as an ultrasonic cutter. Furthermore, in a part of the breakable portion which lay on a distal edge of the lid portion which lay away from a hinge portion of the lid portion which functioned as a hinge when the lid portion was opened and which was provided in the shape of a straight line (a distal edge straight section), a deformation preventive thick part was provided to reduce the deformation of the airbag cover in the vicinity of the distal edge straight section when the airbag is normally pressed (to prevent the occurrence of easy deformation in the airbag cover in the relevant portion thereof at the time of contact), and in order to make this happen, the airbag cover was worked to have a zigzag cross section along the distal edge straight section so that thick portions were provided discontinuously while providing thin portions.
Of course, as airbag covers in which thick portions and thin portions were provided in a distal edge straight section of a breakable portion, there were airbag covers in which thick portions and thin portions were provided discontinuously and integrally with the airbag cover at the time of injection molding without adopting the cutting operation. (refer to, for example, JP-A-5-105016)
With the shape of cross section, however, which can prevent the unnecessary deformation of the airbag cover when it is pressed during the normal use even though the airbag cover is molded by molding such as injection molding and thereafter, the distal edge straight section having the zigzag cross section is formed in the breakable portion by the cutting operation, it is difficult to reduce the breakage load at the distal edge straight section in the breakable portion, and in particular, with a single-layered airbag cover which was formed from a kind of, for example, polyolefin-based synthetic resin material, it was difficult to suppress the breakage load to a low level at the time of low temperatures.
Similarly, with a single layered airbag cover which was produced through only molding such as injection molding, when providing thick portions and thin portions at the distal edge straight section of the breakable portion, even though the thicknesses of the thick portions and the thin portions were made to be substantially the same as those provided by the cutting operation, the breakage load was easy to be increased due to a problem related to the injection molding that the density of the material was increased by the injection pressure.